In high-volume installations for the manufacture of frozen food products, the freezing apparatus frequently comprises a long insulated chamber, called a tunnel, through which the food product passes on an endless belt conveyor. Near the outlet end of the tunnel, the food product is sprayed with a liquified cryogen, frequently liquified nitrogen, as the final freezing step for the product. Cryogen gas, vaporized by contact with the food product and with the conveyor, is directed back through the tunnel in a direction counter to the flow of the food product, progressively chilling the food product as it moves through the tunnel chamber. For effective and consistent operation, it is essential that the food product be subjected to a spray of liquified nitrogen; however, the flow rate for the liquified cryogen spray is regulated to maintain overall thermal conditions within the tunnel within a restricted range and thus avoid excessive wasteful use of the cryogen.
In the course of a normal work shift it may be necessary to interrupt the movement of the food products into and through the freezing tunnel for varying periods and for a variety of different reasons. For example, if the food products being frozen are hamburger patties, any malfunction of upstream equipment may require shutdown of the tunnel for an indeterminate period to permit maintenance personnel to correct the malfunction. Another valid and relatively common reason for a shutdown of freezing tunnel operations is a changeover from one food product to another as, for example, a change from one size hamburger patty to a different size or even a change in the nature of the food product as from hamburger patties to a seafood product such as shrimp or fish.
When the freezing tunnel is shut down for a limited period, as for a food product changeover or for correction of a malfunction of the food product source, the tunnel is maintained at its extremely low working temperature, ready for a new supply of food products. During a shutdown interval of this kind, the thermal regulator for the cryogen input may reduce the flow of cryogen into the tunnel to a very low rate or may even shut off the cryogen flow completely. It is essential that a substantial flow of cryogen into the tunnel be re-established when the tunnel again begins freezing operations. It is also important that the renewed flow of cryogen into the tunnel be accurately timed with respect to the renewed supply of food products; if the cryogen flow rate is increased too soon there may be a substantial waste of the cryogen, whereas if it is not increased soon enough some of the food products may not be properly frozen. Furthermore, startup control is further complicated by the fact that thermal sensors suitable for use at the extremely low temperatures present in a freezing tunnel generally exhibit substantial inertia in operation, making precise timing more difficult than might otherwise be the case.
The latent heat present in food products newly introduced into the tunnel on startup causes a substantial change in thermal conditions within the tunnel on startup. Conversely, an interruption in the supply of food products to the tunnel produces a correspondingly large change in thermal conditions in the tunnel. On shutdown, inertia of the thermal control can cause an appreciable and undesirable waste of cryogen. In general, it has been difficult and sometimes impossible to accommodate short-term changes in the food product supply without substantial loss in efficiency of tunnel operation, either through waste of the cryogen or through failure to achieve proper freezing.